The recent trends toward lighter, smaller, and higher-density electronic products have increased the demand for various printing boards. In particular, the demand for flexible laminates (also referred to as “flexible printing circuit boards (FPCs)”) has shown a notable increase. A flexible laminate is constituted from an insulating film and a circuit formed from a metal foil disposed on the film.
A typical flexible laminate is produced by bonding a metal foil onto a surface of a substrate, with an adhesive material under heating and pressure, the substrate being a flexible film composed of an insulating material of various kinds. Polyimide films and the like are preferred as the insulating flexible film, and thermosetting adhesives such as epoxy and acrylic adhesives are typically used as the adhesive material. Hereinafter, FPCs made using thermosetting adhesives are also referred to as “three-layer FPCs”.
Thermosetting adhesives are advantageous in that bonding at relatively low temperatures is possible. However, requirements for properties, such as heat resistance, flexibility, and electrical reliability, are becoming more stringent, and it is possible that three-layer FPCs using thermosetting adhesives will have difficulty in meeting these stringent requirements. In order to overcome this problem, FPCs (hereinafter also referred to as “two-layer FPCs”) using thermoplastic polyimide as the bonding layer or made by directly forming a metal layer on the insulating film have been proposed. The two-layer FPCs have properties superior to those of the three-layer FPCs, and the demand for the two-layer FPCs is expected to grow in the future.
Various techniques have been proposed to improve adherability of polyimide. For example, a technique of subjecting a polyimide film to a low-temperature plasma treatment and then to modification treatment using alkaline chemicals and a technique of adding a particular metal component to the polyimide film and subjecting the resulting film to high-temperature treatment have been known (see Patent Documents 1 and 2, cited below). However, polyimide films have low adherability to thermoplastic polyimide-based adhesive materials, and the adherability of the polyimide films is still insufficient even after such treatment. Plasma treatment is effective for improving the adherability of the film to the polyimide-based adhesive materials, but sufficient effects cannot be obtained unless a proper resin composition is selected for the polyimide film (see Patent Document 3).
Another approach disclosed is to control the polymerization process of polyamic acid used for the production of the polyimide film so as to control the linear expansion coefficient, the elasticity modulus, and the like of the polyimide film.
For example, the following process is known as the process for preliminarily preparing block components by polymerization: polyamic acid is prepared from phenylenediamine and pyromellitic dianhydride or from phenylene diamine and 3,3′-4,4′-benzophenonetetracarboxylic acid by polymerization to form their block component and subsequently adding an imidizing agent thereto to prepare a copolymerized polyimide containing the block components (see Patent Documents 4 and 5). However, this process uses rigid monomers and the product does not contain any thermoplastic block component. A technique of controlling the block component in view of obtaining high adherability to various adhesives, in particular, polyimide adhesive materials which have had difficulty in adhering to the films prepared by the related art, has not been known so far.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 5-222219
Patent Document 2: Japanese Unexamined Patent Application Publication No. 6-32926
Patent Document 3: Japanese Unexamined Patent Application Publication No. 1-158276
Patent Document 4: Japanese Unexamined Patent Application Publication No. 2000-80178
Patent Document 5: Japanese Unexamined Patent Application Publication No. 2000-119521